The development of a systems model of renal mass transport is proposed. Digital computer simulations of renal transport using this model allows analysis and interpretation of renal physiological data from clearance, micropuncture, and isolated tubule studies. The model structure consists of series of perfectly mixed compartments which represent tubular, vascular, and interstitial spaces with special emphasis on the renal medulla. Mass-balance equations for solutes and water in every compartment will be developed. The associated membrane transport equations will describe solute movement by passive diffusion, solvent drag, and active transport; and water movement by osmosis. The transmural transport parameters for a nephron compartment will be related to experimentally measured membrane transport parameters (based on a tube model of a nephron segment) in a manner which assures that the steady-state responses of the compartment model and tube model are essentially the same. Results from computer simulations using parameter values obtained from the experimental literature will be compared with independently measured experimental data to test hypotheses concerning transport mechanisms in the renal medulla. Two applications of the model will be an investigation of salt transport mechanisms in the inner medulla and an evaluation of data suggesting active urea transport from the collecting ducts. The use of systems modeling techniques should increase the efficiency of experimental research by allowing more complete data analysis and by suggesting experiments needed to distinguish among mechanistic hypotheses.